Such a relief display device is known from U.S. Pat. No. 4,044,350. With this known device a number of piezoelectric flexion members, also called piezoelectric reeds, are supported at their ends and in their central part coupled to a tactile member.
This construction allows a reasonable driving force to be obtained for the tactile member. However, the excursion of the tactile member is rather small with a predetermined flexion of a flexion member of predetermined length.
Devices of the considered type are used as a blind aid for instance in braille cells or rulers or in other devices for tactually displaying a number of points as shown from U.S. Pat. No. 3,229,387.
A difficulty with these devices is that the tactile members have to be located rather near to each other. Due to this and to the fact that the flexion members extend to both sides of the textile members, it becomes difficult to find sufficient space for the flexion members. For instance, in the braille cell shown in U.S. Pat. No. 4,044,350, the members have been mounted above each other in a number equalling the number of braille points in a character (six are shown, but there may be eight in a braille system with further signalling possibilities such as musical notation).
Another braille cell having flexion members has been shown in U.S. Pat. Nos. 2,283,178 and 4,473,356. In these publications the flexion members are mounted by clamping them near to one end in a fitting neck portion. According to U.S. Pat. No. 4,283,178, the flexion member is activated by a DC voltage and in U.S. Pat. No. 4,473,356 by an AC voltage. It should be noted that with the present invention both types of excitation are possible, because if a person can sense whether the tactile member is in the first or the second position he will also be able to sense its vibrational movement between the first and the second positions.
A difficult of these two publications is, however, that the flexion members or reeds are clamped at one of their ends. This feature at the one hand allows for a considerable excursion with a certain flexion member but leads to relatively small driving forces for the tactile members.
A further disadvantage of these known devices is, that with a predetermined force to be exerted on the tactile member, a rather srong flexing momentum works at the flexion member in the immediate proximity of its clamped part.
Still a further disadvantage is that it is very difficult to have the flexion member mounted in the correct position. This is due to the fact that angular displacements of the flexion members at their clamping location are multiplied by almost the whole length of the flexion members, whereas it is not sure that all flexion members have exactly the same shape when no exciting voltage is applied to them. The above reasons necessitate an individual adjustment, for instance of the length of the tactile members, which means additional labour and a complication of the construction. Also, reed adjustment during use at times may be necessary. Individually adjusted tactile members have the further disadvantage that with cleaning, for instance in order to remove cutaneous fat, one has to see to it that the same tactile member again cooperates with the same flexion member after the tactile members have been temporarily demounted.
Finally, it is remarked that increase of the flexion excursion or the driving force for the tactile member by augmenting the applied voltage may be undesired or even forbidden in view of safety regulations.